1. Field of the Invention
The present invention relates to a method of automatic level matching in a local network, particularly for a multicomputer arrangement, comprising a bus system having light waveguides and an optical star coupler, for setting a standardized level at all outputs of the star coupler for the purpose of collision recognition, whereby the information signal to be transmitted is converted into an optical signal by an electro-optical transmitter, the optical signal being coupled in a light waveguide. The optical signal transmitted via the light waveguide is converted into an electrical useful signal by an opto-electric receiver and the information signal is recovered from the electrical useful signal. A respective transmitter is provided for each node, for example, a computer, the light power which is dependent on the information signal and which is coupled into the light waveguide by the transmitter being variable in digital steps. A receiver constructed of DC-coupled amplifiers without gain control is respectively provided for each node, the receiver comprising an input stage for generating the useful signal, a first comparator stage for generating the information signal from the useful signal and a second comparator stage which is supplied with a reference voltage variable in digital steps which senses the level of the useful signal and emits a collision signal when the useful signal exceeds the reference voltage.
2. Description of the Prior Art
Among known local networks there are arrangements in which computer nodes communicate with one another via a bus system, for example an optical bus system having a star coupler. The bus access is controlled by various techniques which can be divided into two main groups; deterministic methods which make collisions impossible, for example time division multiple access (TDMA), and stochastic methods which permit collisions, for example carrier sense multiple access collision detection (CSMA/CD). Recognizing a collision as simply as possible, reliably and quickly is a problem existing given stochastic methods. It is essential that all nodes be able to recognize a collision, even those nodes which are not participating in a collision.
A collision recognition of the prior art can occur by information comparison, by phase/transit time comparison or by level comparison.
In the "SIELOCnet" (Siemens local network), the computer nodes, for example, communicate with one another via an optical bus system. In terms of structure and operation, the optical bus system must be adapted to the "SIELOCnet" requirements and objectives and must promote the same. Only in this manner can one obtain a logically-uniform, straightline overall system which can also fully develop the designed performance capability.
The optical bus system for this known local network is composed of an optical transmission system and of a network controller (NC). Among other things, the network controller assumes the control of the data transfer between the optical transmission system and a host computer, as well as the editing, parallel-to-serial conversion, coding and formatting of the data.
The optical transmission system is divided into the transmission path, the optical network and the transmitreceive module (known as "transceiver" in local networks). The optical network must do justice to the requirement for decentralized communication, i.e. request controlled. Moreover, it must be possible to modify the network configuration without interrupting operations in that the number of computer nodes is increased or decreased. For this purpose, it is possible to execute the optical network as an optical bus with a star coupler (forward mixer). The transceiver function is realized in a bus interface connection which, among other things, also assumes the modification and the optical-electrical conversion of the serial data signals of the network controller.
The available optical transmission system operates with adjustable electro-optical transmitters and DC-coupled, opto-electrical receivers. With respect to signaling, monitoring, coding, transmission speed and freedom from disruption, therefore, both the DC character of the transmission system and the particular advantages of the light waveguide (LWL) are exploited, in particular the insensitivity to electromagnetic disturbances, separation of potential between transmitter and receiver, no signal radiation and, as a result, no crosstalk, no sparking at fiber contact locations or, given fiber breaks, no ground loops, high bandwidth and low weight.
The star coupler employed given an optical bus can be a forward mixer or a reflection mixer. Given the forward mixer, all inputs are located at the one end face and all outputs are located at the other end face of the quartz laminae, the actual mixer. In the reflection mixer, one end face of the mixer laminae is mirrored. The inputs and outputs of the mixer are all placed at the second face of the laminae. Therefore, the laminae must be widened in this case and must be lengthened for a degree of mixing of equal quality. As a consequence of the mirror and the larger laminae, the losses are greater than given a forward mixer. Forward mixers are employed for "SIELOCnet".
The logical bus function is realized in the mixer laminae. The optical signal which has proceeded from a computer node to an input of the mixer laminae is uniformly distributed to all outputs. Given utilization of a star coupler, the length of the optical bus is diminished to a point. The transmission lines s.sub.1. . . s.sub.n and the receiving lines e.sub.1. . . e.sub.n are viewed as leads to the optical bus. Topologically, an optical bus with a star coupler therefore looks like a star but is logically a true bus, cf. FIG. 1.
Only two hosts computers can communicate with one another simultaneously over the optical bus. The communication desires of the host computers must therefore be synchronized. The five known phases fundamentally apply for the transmission of a packet of packet-oriented data transmission on which the following is based: connection set up, beginning of the data transmission, data transmission, end of the data transmission and connection cleardown.
Given a bus access, the bus seizure corresponds to the connection set up and the bus release corresponds to the connection clear down. The bus access is controlled by the various methods set forth above. Independently of the bus access method, it is essential to allocate an unequivocal physical criterion to the respective bus states of busy or free. In the cases of a transmission with light waveguides, EQU Continuous light.sigma.Bus Occupied EQU No light.sigma.Bus free
Due to the DC character of the optical transmission system, the coding and the character format can be liberally selected for the data transmission. An asynchronous character format, for example, is defined for "SIELOCnet". The data transmission cannot begin until the bus is occupied. The state "bus occupied" must therefore correspond to the quiescent condition for the data transmission. Related to the character format, that means: continuous light corresponds to "stop polarity".
The beginning of the data transmission is characterized by the start bit of the first character to be transmitted after the bus seizure. The time duration from the bus seizure up to the first start bit is freely selectable and is defined in the transmitter circuit of the network controller. This once-defined time duration is monitored and evaluated by the receiver circuit. The function of the serialto-parallel conversion is only started when the first start bit arrives within the monitoring interval. In this manner, variable light pulses which are longer than the monitoring interval can be transmitted for signaling purposes without starting the actual receiver function.
The end of the data transmission is identified by the non-arrival of further start bits. It is assumed that the transmitter circuit transmits all characters of a packet in gap-free succession. That is also indispensible for a good exploitation of the data channel. After the last character of a packet "stop polarity" must prevail as a packet and identification for a sufficiently long interval. Only then is the bus released. The same timer stage which is also utilized for the evaluation of the beginning of a data transmission can be employed for the evaluation.
FIG. 2 illustrates the block frame of a data packet which meets both the requirements of an asynchronous transmission procedure as well as the requirement of a transmission protocol for an optical bus system. This definition spans the entire optical transmission system. The functions "block recognition", "data transmission", "collision recognition", and "special signaling" can be realized as a common concept.
In stochastic bus access methods, collisions occur when two or more computer nodes send a data packet roughly simultaneously (dependent on the system transit time). Given an optical bus with star couplers, a collision of data packets can occur only in a punctiform manner in the star coupler, since, of course, each computer node has a separate light wave guide assigned thereto for transmission and for reception, cf. FIG. 1.
A collision recognition can occur by way of level comparison, phase and transit time comparison or information comparison. A method for collision recognition should be realizable as independently as possible of the coding and should be realizable on the physical level of the "ISO layer model" of the ISO (international standards organization) in order to avoid complex algorithms in higher levels. As the most simple and straight line possibility, collision recognition by way of level comparison was selected for "SIELOCnet". This method is predestined for an optical bus with star couplers because collisions can only appear in a punctiform manner and the optical levels are falsified neither by in-scatterings nor by ground loops.
When a collision occurs, the corresponding optical levels add in the star coupler. This signal mixture reaches every computer node, even those computer nodes which are not participating in the collision. By evaluating this mixed level, each computer node directly recognizes a collision without a special signal ("jam signal") having to be additionally sent first to all computer nodes which are likewise not participating in the collision. A distinction is made between single level and two through n-fold levels. For a clear and reliable evaluation of a double level over a single level, the light levels occurring from arbitrary transmitters must appear of identical size for a defined receiver. For this reason, the optical transmission system must be matched. Due to the DC character of the optical transmission system, the sum levels can thereby be transmitted unfalsified. Linearity is necessary at least up to twice the value of the maximum, single level.
Given a collision of two data packets, the concept of collision recognition via level evaluation requires a level addition at least for the duration of a data bit, namely a level addition independently of the chronological relationship of the packets relative to one another and of the data content of the packets. In addition to the actual information, criteria which enable the receiver to produce a phase reference in the evaluation of the bit stream must also be transmitted in the serial data transmission. This can be achieved by auxiliary information (for example start-stop bit, SYN character) or code manipulation (for example scrambler, Manchester code). What is shared by all measures is that they must be cyclically repeated. Only in this manner can the phase reference be maintained during the entire duration of the data transmission. The repetition cycle is of different length in the individual measures and amounts, for example, in the manchester code to one bit, amounts of one character in the start/stop method, is dependent on the selected polynomial given use of a scrambler, and the stability of the clock generator is a crucial factor given utilization of SYN characters. When only the cyclically-reoccurring synchronization measures are taken into consideration for a level addition, then one is independent of the data content of the packets. The low addition always occurs when a continuous light pulse having the length of the repetition cycle of the appertaining synchronization measure superimposes with a data packet. This principle guarantees a collision recognition with 100% probability.
It is most advantageous to precede each data packet with a continuous light pulse of corresponding lengths. A level addition then occurs immediately at the beginning of a collision and is therefore recognized as early as possible. This agrees with the previous determination: therefore, occupation equals continuous light. On the basis of the selected asynchronous character format, the continuous light pulse at the beginning of the data packet is defined with 11 bits (about 1 character) length. Given a collision, a level addition of at least one bit length occurs when the bit stream is composed only of stop bits, i.e. the data are "OOH". The end of the data packet is marked with 20 bits (2 characters) continuous light. When a bus access from a computer node only occurs given a free bus ("carrier sense" function) and when the data packets are longer than the transmit time on the optical bus, then a collision for data packets can begin only at the beginning or during a data packet. When the data packets are shorter than the transit time on the optical bus or when the data bus access is arbitrary, a collision can also begin at the end of a data packet. The most universal case is to be covered for the collision recognition because the data packets are framed with continuous light pulses.